1. Field
The present disclosure relates to synthesis of ultra-large graphene oxide (UL-GO) sheets. More particularly, the disclosure relates to the application of UL-GO sheets for a Langmuir-Blodgett (LB) assembly technique to uniformly deposit the sheets onto both rigid and flexible substrates for producing optoelectronic thin films.
2. Background
Graphene, which consists of a two-dimensional monolayer of sp2-bonded carbon atoms, has received tremendous attention due to its unique transporting properties. It finds a rapidly growing application in the fabrication of transparent conductors. A key to success in such an application is to develop methods for producing large-size graphene sheets with high yields. The graphene sheets in current use for the fabrication of transparent conductors are very small, mostly with an area of hundreds of square micrometers at best. The large number of intersheet junctions between small-size graphene sheets leads to high intersheet contact resistance. Another key issue is how to deposit the graphene sheets onto the substrate uniformly in an aligned manner. Several well-established deposition techniques, including spin/spray coating, transfer printing, dip coating, electrophoretic deposition, and Langmuir-Blodgett (LB) assembly have been developed with varied success. Among these approaches, the LB assembly is the only technique that can realize controllable deposition of ultra-large graphene oxide (UL-GO) in a layer-by-layer manner. The thickness of GO films can be accurately controlled upon repeated deposition, leading to optimized optical and electrical properties of the final products.
Several methods are designed to directly synthesize graphene on a laboratory scale, like mechanical cleavage and unzipping carbon nanotubes, and the suitability of these methods for mass production remains unproven. Chemical vapor deposition (CVD) is currently the most popular approach for synthesizing graphene, which usually requires specific substrate materials that have to be removed chemically after the growth of graphene. The high cost of single crystal substrates and the ultrahigh vacuum conditions necessary to maintain the CVD growth significantly limit the use of the method for large-scale applications. In contrast, GO has been considered an important precursor for the fabrication of graphene owing to the scalability of production and the convenience in processing.
Previous techniques have been unable to fully exfoliate, leaving many unreacted graphite flakes. As a result, the film prepared using these GO sheets gives a transmittance of 78% or less.
The fabrication of GO involves exfoliation and oxidation of natural graphite (NG) flakes using potassium and one or more concentrated acids. However, previous techniques were either unable to fully exfoliate NG, or using destructive ultrasonication or long mechanical shaking that often result in severe damage and fragmentation of GO sheets. As a consequence, the vast majority of GO sheets produced in the past is reported to be very small in their sizes with an average area smaller than 1-10 μm2; and the film prepared using these GO sheets gives a high sheet resistance with a transmittance of 78% or less. Aiming at fabrication of high-quality, large-size monolayer GO sheets with high yields, the alteration of oxidation and exfoliation, and use of high quality NG flakes are carefully investigated.